POST-CAREER DEPRESSION: Andre Waters (number 20)--seen here being attended to for a leg injury during the 1992 season--suffered several concussions throughout his NFL career. In 2006, Waters committed suicide.
AP Photo/Wilfredo Lee

Former Philadelphia Eagles star defensive back Andre Waters was known as a fierce tackler during his 12 seasons. By the time he retired in 1995, he had racked up hundreds of tackles but had also sustained numerous concussions.

After his playing days were over, he was reported to be suffering from depression. And in 2006, at age 44, he committed suicide with a gunshot to his head. According to forensic pathologist Bennet Omalu of the University of Pittsburgh, an autopsy after his death revealed that Waters' brain had suffered so much damage from football injuries that it resembled that of an 85-year-old man with early stage Alzheimer's disease. Omalu told The New York Times that he believes the depression and brain damage resulted from his career-related head injuries.

A 2007 study by the Center for the Study of Retired Athletes (CSRA) backs his findings. According to the research, published in the journal Medicine and Science in Sports and Exercise, National Football League (NFL) players surveyed who had sustained three or more concussions were three times as likely to develop clinical depression as players who had not suffered concussions. An earlier study in the Journal of Neuroscience showed that this group was also five times more likely to develop mild cognitive impairment—a condition linked to neurodegenerative diseases such as Alzheimer's.

NFL players pride themselves on clobbering their opponents. But their punishing high-speed hits come with an added risk of sustaining concussions.

Concussions occur when force from an impact or sudden motion jostles the brain, causing it to smack against the surrounding skull or rotate unnaturally. Symptoms include nausea, proverbially "seeing stars" and, in some cases, concussions may cause unconsciousness, memory loss and slurred speech. Once believed to be relatively innocuous in most cases, scientists now fear that repeated concussions may lead to debilitating neurological damage.

Since 2001, the NFL Players Association (NFLPA) has partnered with the CSRA—based at the University of North Carolina at Chapel Hill (U.N.C.)—to determine whether pro ball players suffer any lingering health effects from years of being knocked around on the field. The NFL has criticized the findings, charging that they're unreliable because they are based on self-reporting by players. In an effort to address this concern, the CSRA double-checked players’ responses with close confidants and re-surveyed others up to two years later to see if their answers were consistent.

ScientificAmerican.com spoke to CSRA's program director Kevin Guskiewicz about the possible link between head trauma and depression and other mental health problems, as well as a new study assessing pituitary gland dysfunction in 90 former NFL players. An edited transcript of the conversation follows:

According to your data, how frequently do football players get concussions?
In the [2007] survey of 2,552 retired players, almost 61 percent in the sample indicated that they had [suffered] a concussion in their career. Of that, a significant number [595] had three or more.